Recent OS X updates (SIP) have rendered some old solutions (especially scripts) broken. Should old answers be updated, or new questions answered in their own right?
The old solutions are not broken; the new operating systems are :-)
The posted solutions are still quite valid for computers running the operating systems for which the answers were written. I don't see any reason to break the answers for people running Panther or Tiger or Leopard or Snow Leopard or Lion or Mountain Lion or…
If one wanted to edit additional information into the answers to make them compatible with these newfangled operating systems that aren't named after cats, I suppose one could do that, but don't break answers that work fine for the systems for which they were intended.
If there is a clearly-winning, correct answer which only changes at for instance El Capitan, my inclination would be to just add something along the lines of
"Edit: after El Capitan you need to do this instead/as well"
which preserves the original & updates for newer systems and also keeps the right answer for all occasions at the top of the list.
It also keeps the rep points accruing for the person who actually got the right answer in the first place; it wasn't their fault El Capitan broke their elegant solution.